s when the electric circuit is broken by a projectile. Another
method is to arrange the terminals of the secondary circuit of an
induction coil, so that when the primary circuit is opened a small spark
punctures or marks a moving surface (Helmholtz, _Phil. Mag._, 1853, p.
6). A photographic plate or film, moving in a dark chamber, is also used
to receive markings produced by a beam of light interrupted by a small
screen attached to an electromagnetic stylus, or by the legs of a
tuning-fork, or by the mercury column of a capillary electrometer. In
certain researches on the explosive wave of gases the light given by the
burning gases made the time trace on a rapidly moving photographic film
(H.B. Dixon, _Phil. Trans._, 1903, 200, p. 323). In physiological
chronography the stylus is in many cases actuated directly by the piece
of muscle to which it is attached; when the muscle is stimulated its
contraction moves the stylus on the moving surface of the myograph (M.
Foster, _Text Book of Physiology_, 1879, p. 39).

Grobert and Dabooz.

_Gun Chronographs._--Probably the earliest forms of chronographs, not
based on the ballistic pendulum method, are due to Colonel Grobert,
1804, and Colonel Dabooz, 1818, both officers of the French army. In
the instrument by Grobert two large disks, attached to the same axle
13 ft. apart, were rapidly rotated; the shot pierced each disk, the
angle between two holes giving the time of flight of the ball, when
the angular velocity of the disks was known. In the instrument by
Colonel Dabooz a cord passing over two light pulleys, one close to the
gun, the other at a given distance from it, was stretched by a weight
at the gun end and by a heavy screen at the other end. Behind this
screen there was a fixed screen. The shot cut the cord and liberated
the screen, which was perforated during its fall. The height of fall
was measured by superposing the hole in the moving screen upon that in
the fixed one. This gave the approximate time of flight of the shot
over a given distance, and hence its velocity.

Wheatstone.

In the early form of chronoscope invented by Sir C. Wheatstone in 1840
the period of time was measured by means of a species of clock, driven
by a weight; the dial pointer was started and stopped by the action of
an electromagnet which moved a pawl engaging with a toothed wheel
fixed on the axle to which the dial pointer was attached.